1. Field of the Invention
The present invention broadly relates to combustion stability in the afterburner system of a jet aircraft engine and, more particularly, is concerned with a device for providing a more stable flame in the fan stream region of the afterburner during augmentor operation in a turbofan jet engine.
2. Description of the Prior Art
In order to increase the effective thrust of the common turbojet engine, bladed fans have been added to a turbine-driven shaft thereof to effect the flow of a quantity of atmospheric air through an annular passage defined between the turbine and a radially spaced casing added thereto. The turbofan engine, as this combination has come to be known, has been found to be more efficient if the hot gas stream flow from the core engine (the basic turbojet portion of the turbofan) and the fan stream (the air stream forced through the annular passage by the fan) are mixed together before expulsion through a single discharge nozzle.
To develop still more thrust for takeoff and climb and for periods of dash of the aircraft, it is also advantageous to augment the engine thrust by burning additional fuel in an afterburner system. The afterburner system would be located in the turbofan engine between the turbine and the discharge nozzle at a desired location for both mixing and combustion of the hot gas stream flow from the core engine and the fan stream flow from the annular passage leading from the bladed fan.
It has been the practice in afterburner systems to use fixed baffles as flameholders at such mixing and combustion location. Baffles produce stable turbulent (or recirculation) zones wherein flames will maintain themselves. However, such baffles offer undesirable resistance to flow during periods when afterburning is not in operation. Thus, there is a tradeoff involved in using an afterburning system to augment thrust.
One of the more common forms of the fixed baffle is a V-shaped gutter having its apex pointing upstream of the flow. By setting the legs of the gutter to an included angle on the order of 45 to 60 degrees to flow velocity, a reasonably optimum tradeoff between resistance and the provision of an adequate recirculation zone can be obtained. However, when cold augmentor fuel droplets fed into the fan stream flow contact the baffle, the latter is cooled to the extent that smooth combustion of the augmentor fuel in the afterburner is impeded. Instead, an unevenly burning flame results which causes a low frequency rumble in the afterburner system as well as low efficiency combustion of the augmentor fuel. Therefore, a need exists for a technique to shield the flameholder baffle from contact by cold fuel while improving the delivery of the fuel to the recirculation zone wherein the flame is maintained.